When most people talk about solar power in the U.S., they tend to think narrowly about photovoltaics. But it’s a big world out there. Even in the U.S., researchers are working on interesting alternatives, particularly for big commercial bulk power facilities intended as alternatives to nuclear or fossil-fuel power plants.
One of the issues with photovoltaics is just how far they can scale. The largest photovoltaic plant in the U.S., Florida Power and Light’s 25-MW Next Generation Solar Energy Center in DeSoto County, comprises more than 90,000 SunPower solar panels on 180 acres. The world’s largest photovoltaic plant, the 60-MW Olmedilla Photovoltaic Park (Fig. 1) in Olmedilla de Alarcón, Spain, uses more than 160,000 conventional solar photovoltaic panels.
Olmedilla may represent a limit on the maximum capacity of photovoltaic solar. Or it may not. These technologies have a way of leapfrogging each other. Recently, First Solar in Tempe, Ariz., announced that it would build a 2000-MW solar power plant in China. First Solar makes thin-film solar cells in a continuous process, rather than on wafers. However, its cells use tellurium, the relative scarcity of which could limit product volumes.
Efficiency is another issue related to scaling. In terms of wafer-based cells, SunPower offers a conversion ratio of 23.4%, while the average for the rest of the market ranges from 12% to18%. SunPower’s efficiency comes from metalizing the collection grid on the back of the wafer, rather than the front, as it is on other manufacturers’ wafers. Back-collection increases the photon-capture area of the wafer. What makes this possible is SunPower’s process technology, which allows the charge carriers created by the solar-cell junctions when they are hit by photons to migrate all the way through the thickness of the wafer.
However, Sharp Solar has recently announced cells with 35.8% conversion efficiencies using a triple-junction compound solar cell. (Multiple band-gaps allow the junctions to respond to a broader range of photon energy levels—that is, a broader light spectrum.) This approach has been used for decades in powering expensive military satellites, but the manufacturing cost put it out of reach for commercial use.
Beyond that, experimenters at DuPont and the University of Delaware have achieved 42% efficiencies by using multiple band-gap collectors and spectral-splitting optics, according to the university.
Until FirstSolar’s Chinese photovoltaic plant is built, photovoltaics seem limited to a scale of tens of megawatts and hundreds of acres. At the same time, as production economies of scale kick in, photovoltaic panels appear to be the ideal fit for what the Smart Grid visionaries call “distributed resources,” domestic rooftop systems like the 2.5 kW on my own roof, or the distributed 2.5 MW that Intel announced it would install in the first half of 2010 on factory roofs in Arizona, California, New Mexico, and Oregon.
LINEAR CONCENTRATED SOLAR
Non-photovoltaic methods can be lumped into the concentrated solar thermal (CST) category. The common approaches include linear, power tower, and dish-engine systems.
Linear systems generally comprise multiple parallel rows of solar collectors. They may be stationary, or they may be motor-driven to pivot around a north-south axis, following the sun from east to west through the day. Parabolic trough systems can focus sunlight by 30 to 100 times, heating a fluid in a pipe that runs down the focal line of each trough. A heat exchanger then boils water to produce superheated steam to run turbine generators. It’s possible to run the same turbines at night or during cloudy days using natural gas as a fuel, providing continuous capacity.
Linear systems are sometimes used with thermal energy storage (TES) systems. This typically involves a salt mixture of 60% sodium nitrate and 40% potassium nitrate, which melts at 221°C. The TES system keeps the mixture as a liquid and cycles it between “cold” storage at 288°C and “hot” storage at 566°C.
In the solar plant, the hot salt is used at night to take over the job of superheating steam for the turbine generator. A 100-MW turbine can be run for four hours by pumping molten salt between hot and cold tanks roughly 30 feet tall and 80 feet in diameter.
Linear Fresnel-reflector concentrating systems are an alternative to troughs. They employ flat or slightly curved mirrors and Fresnel lenses to concentrate sunlight onto a pipe. They aren’t as optically efficient as parabolic troughs, but they can be cheaper because they can heat water for the turbines directly. They also require less land because design engineers don’t have to worry about one trough shading another.
The largest non-photovoltaic installation (Fig. 2) in the world is a collection of nine trough systems in California. The Solar Energy Generating Systems (SEGS) 364-MW collection of nine solar-trough power plants, operated and partially owned by NextEra Energy Resources, comprises SEGS I-II (44 MW), located at Daggett, SEGS III-VII (150 MW), located at Kramer Junction, and SEGS VIII-IX (160 MW), located at Harper Lake, all near Barstow, Calif., in the Mojave Desert. Taken together, the nine plants cover almost eight square miles.